Homoepitaxial gallium nitride based photodetector and method of producing

ABSTRACT

A photodetector ( 100, 200, 300 ) comprising a gallium nitride substrate, at least one active layer ( 104, 302 ) disposed on the substrate ( 102, 202, 306 ), and a conductive contact structure ( 106, 210, 308 ) affixed to the active layer ( 104, 302 ) and, in some embodiments, the substrate ( 102, 202, 306 ). The invention includes photodetectors ( 100, 200, 300 ) having metal-semiconductor-metal structures, P-i-N structures, and Schottky-barrier structures. The active layers ( 104, 302 ) may comprise Ga 1-x-y Al x In y N 1-z-w  P z As w , or, preferably, Ga 1-x Al x N. The gallium nitride substrate comprises a single crystal gallium nitride wafer and has a dislocation density of less than about 10 5  cm −2 . A method of making the photodetector ( 100, 200, 300 ) is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/839,941,filed Apr. 20, 2001 now U.S. Pat. No. 6,806,508, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to crystalline gallium nitride. In particular, theinvention relates to a homoepitaxial gallium nitride based photodetectorand a method of producing the same.

During the past decade there has been tremendous interest in galliumnitride (GaN) based optoelectronic devices, including, for example,light emitting diodes (LEDs) and laser diodes (LDs). Becausehigh-quality GaN substrates have not been available, virtually all ofthe art has involved heteroepitaxial deposition of GaN and GaInAlN onsapphire or SiC substrates. A thin low-temperature buffer layer,typically AlN or GaN, is used in order to accommodate the latticemismatch between GaN and the substrate and maintain an epitaxialrelationship to the substrate.

Several processes are currently used to produce crystalline galliumnitride substrates. The processes include heteroepitaxial growth ofgallium nitride on a substrate, such as a sapphire or silicon carbide.The heteroepitaxial growth process often results in defects includinghigh concentrations of dislocations, vacancies, or impurities. Thesedefects may have undesirable and detrimental effects on epitaxiallygrown gallium nitride, and may adversely influence operation of theresultant gallium nitride-based device. These adverse influences includecompromised electronic performance and operation. Presently,heteroepitaxial gallium nitride growth processes require complex andtedious steps to reduce defect concentrations in the gallium nitride.

Known growth processes do not provide large gallium nitride crystals ofhigh quality (i.e.; crystals having low dislocation densities); forexample, gallium nitride crystals greater than about 0.8 inches (about 2centimeters) in diameter or greater than about 0.01 inches (about 250microns) in thickness. Further, the known methods are not known toprovide for production of large gallium nitride crystals that result insingle-crystal gallium nitride boules, for example gallium nitridecrystals of about 1 inch in diameter and about 0.5 inches in thickness,which are suitable for forming wafers. Thus, applications for galliumnitride are limited due to size constraints.

Known methods of producing large-area GaN wafers yield wafers havingrather high (>10⁶ cm⁻²) concentrations of threading dislocations. As isthe case in heteroepitaxial devices, high concentrations of such defectsdegrade device performance.

Also, most known gallium nitride crystal production processes do notprovide high-quality gallium nitride crystals with low concentrations ofimpurities and dislocations with adequate size and growth rates that areacceptable for device applications. Further, the known gallium nitridecrystal production processes are not believed to provide an economicalprocess having nitride growth rates that enable moderate-cost galliumnitride crystal production. Therefore, applications for gallium nitrideare further limited due to quality and cost-of-production factors.

Use of gallium nitride crystal has been limited in photodetectorapplications because of the quality and manufacturing issues discussedabove. A high-performance photodetector could be used, for example, tocontrol the temperature in the combustor of power-generation turbines orin aircraft engines, allowing continuous, real-time optimization ofcombustion conditions and improved energy efficiency and reliability.Photodetectors could also be used in a wide variety of sensorapplications, both civilian and military. While current buffer-layertechnology allows for production of commercially viable GaN-based LEDsand LDs, the photodetectors that can be produced with current technologyare marginal in performance because of very high defect levels.

Growth of homoepitaxial photodetectors on high-quality GaN substrateswould offer improved sensitivity, increased efficiency, reduced leakage(dark) current, and increased breakdown field. Other potential benefitsof homoepitaxial photodetectors include increased temperature ofoperation, better reliability, better device uniformity, improvedbackside contact capability, higher manufacturing yield, longerlifetime, enhanced wafer utilization, improved wavelength selectivity,and better manufacturability.

Accordingly, there is a need in the art for an improved GaN basedphotodetector.

BRIEF SUMMARY OF THE INVENTION

The present invention meets this need and others by providing aphotodetector having a gallium nitride substrate, a gallium nitridesubstrate for a photodetector device, and a method of producing such aphotodetector.

The photodetector of the present invention includes a gallium nitridesubstrate, at least one active layer disposed on the substrate, and aconductive contact structure affixed to the active layer and, in someembodiments, the substrate. In one embodiment of the invention, thephotodetector has a metal-semiconductor-metal (MSM) type structure, inwhich an insulating active layer is deposited on the gallium nitridesubstrate, and the conductive contact structure is a patterned array ofinterdigitated Schottky-type (i.e., rectifying) metallic contactsconnected to the semi-insulating active layer.

Another embodiment of the invention is a photodetector having a P-i-Nstructure. The photodetector includes either an n-doped gallium nitridesubstrate or an n-doped active layer deposited on the substrate, aninsulating active layer, and a p-doped active layer. In this embodiment,the conductive contact structure comprises at least one ohmic-typecontact connected to the p-type active layer and an ohmic contactconnected to the substrate.

The photodetector of the present invention also encompasses a thirdembodiment, which is a Schottky-barrier structure, in which aninsulating active layer is deposited on the gallium nitride substrate,and the conductive contact structure comprises at least oneSchottky-type contact connected to the insulating active layer and anohmic contact connected to the substrate.

Accordingly, one aspect of the present invention is to provide aphotodetector comprising: a gallium nitride substrate; at least oneactive layer disposed on the gallium nitride substrate; and at least oneconductive contact structure affixed to at least one of the galliumnitride substrate and the active layer.

A second aspect of the present invention is to provide a gallium nitridesubstrate for a photodetector. The gallium nitride substrate comprises asingle crystal gallium nitride wafer and has a dislocation density ofless than about 10⁵ cm^(−2.)

A third aspect of the present invention is to provide a photodetector.The photodetector comprises: a gallium nitride substrate, the galliumnitride substrate comprising a single crystal gallium nitride wafer andhaving a dislocation density of less than about 10⁵ cm⁻²; at least oneactive layer disposed on the gallium nitride substrate, the active layercomprising Ga_(1-x-y)Al_(x)In_(y)N_(1-z-w)P_(z)As_(w), wherein 0≦x, y,z, w≦1, 0≦x+y≦1, and 0≦z+w≦1; and at least one conductive contactstructure affixed to at least one of the gallium nitride substrate andthe active layer.

A fourth aspect of the invention is to provide a method of making aphotodetector, the photodetector comprising a gallium nitride substrate,at least one active layer disposed on the gallium nitride substrate, andat least one conductive contact structure affixed to at least one of thegallium nitride substrate and the active layer. The method comprises thesteps of: providing a gallium nitride substrate; depositing at least oneactive layer on the gallium nitride substrate; and affixing a conductiveconnecting structure to at least one of the at least one active layerand the gallium nitride substrate.

These and other aspects, advantages, and salient features of theinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

LIST OF FIGURES

FIG. 1 is a schematic depiction of a prior art photodetector;

FIG. 2 is a schematic depiction of a photodetector in accordance withone embodiment of the instant invention;

FIG. 3 is a schematic depiction of a photodetector in accordance withanother embodiment of the instant invention;

FIG. 4 is a schematic depiction of a photodetector in accordance withanother embodiment of the instant invention; and

FIG. 5 is a flow chart depicting method steps in accordance with oneembodiment of the instant invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, like reference charactersdesignate like or corresponding parts throughout the several views shownin the figures. It is also understood that terms such as “top,”“bottom,” “outward,” “inward,” and the like are words of convenience andare not to be construed as limiting terms.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing a preferred embodiment of the invention and are not intendedto limit the invention thereto. FIG. 1 depicts a prior art GaN/AlGaN onsapphire mesa p-i-n photodetector 10. Photodetector 10 comprises asapphire substrate layer 11, an n-type AlGaN ohmic contact layer 12, anintrinsic GaN absorption layer 13, and a p-type GaN ohmic contact layer14. The sapphire substrate layer 11 is transparent to the GaN absorptionlayers so it can detect the optical field from the backside. Layer 12 iscomprises an n-type AlGaN layer and an n-type metal contact layer, suchas Ti/Al/Ti/Au, which is deposited onto the AlGaN layer and annealed toform an ohmic contact. Layer 14 comprises a p-type AlGaN layer and ap-type metal layer, such as, but not limited to, Ni/Au, which isdeposited onto the AlGaN layer and annealed to form an ohmic contact.The photodiode is operated by reverse biasing the junction. This is doneby applying a positive voltage to the n-type metal contact layer 12.Under these conditions, the current is approximately independent of thevoltage, but is proportional to the rate of optical generation ofcarriers. Layer 13 is the GaN intrinsic region where the photons areabsorbed and generate electron/hole pairs that are drawn to oppositessides of the junction by the electric field where they then contributeto the detector current.

In accordance with one embodiment of the instant invention, shown inFIG. 2, an exemplary embodiment of a metal-semiconductor-metal (MSM)photodetector 100 comprises a gallium nitride substrate 102, at leastone active layer 104 disposed on the gallium nitride substrate 102, andat least one conductive contact structure 106 affixed to the at leastone active layer 104, as shown in FIG. 2. In one embodiment, at leastone active layer 104 comprisesGa_(1-x-y)Al_(x)In_(y)N_(1-z-w)P_(z)As_(w), wherein 0≦x, y, z, w≦1,0≦x+y≦1, and 0≦z+w≦1. In another embodiment, at least one active layer104 comprises Ga_(1-x)Al_(x)N, wherein 0≦x≦1.

In one embodiment, the GaN substrate for the device fabrication consistsof an (0001)-oriented GaN wafer cut from a boule that was grown using asupercritical solvent at a temperature greater than about 550° C. and apressure greater than about 5 kbar.

More specifically, one suitable process for forming the GaN substratecomprises providing a source gallium nitride, solvent, and mineralizer.The source gallium nitride may comprise at least one ofpoorly-crystallized gallium nitride, well-crystallized gallium nitride,amorphous gallium nitride, polycrystalline gallium nitride, andcombinations thereof. The source gallium nitride may be provided “as-is”in its raw form. Alternatively, the source gallium nitride can becompacted into a “pill” or sintered into a polycrystalline compact.Alternatively, the source gallium nitride can be formed in situ byproviding gallium metal which then reacts with the ammonia solvent aftersealing of the capsule and treatment at high pressure and hightemperature to form source gallium nitride.

The source gallium nitride may then be combined with at least one of themineralizer and solvent to form a mixture. The gallium nitride, solvent,and mineralizer may optionally be provided individually to the capsuleas separate and distinct un-combined materials. The mixture, which cancomprise gallium nitride and at least one of the solvent andmineralizer, can be optionally compacted into a pill, however thecompacting of the mixture need not be conducted in the gallium nitridegrowth process.

The source gallium nitride, solvent, and mineralizer are then placedinside a capsule as either a compacted or uncompacted mixture.Optionally, additional mineralizer can also be added to the capsule. Thecapsule, which will be described hereinafter, can then be filled with anitrogen-containing solvent, for example at least one of ammonia orhydrazine, or an organic solvent, including but not limited to,methylamine, melamine, ethylene diamine, and mixtures thereof. Thecapsule is then sealed, disposed in a pressure cell, and subjected tohigh pressure and high temperature conditions in an appropriate highpressure high temperature (HPHT) system. The HPHT conditions aremaintained for a length of time sufficient to dissolve the sourcegallium nitride and re-precipitate it onto at least one gallium nitridecrystal, gallium nitride boule, or gallium nitride crystal seed.

Maintaining HPHT conditions yields large single gallium nitridecrystals, for example single gallium nitride crystals having a diameterand thickness in a range from about 0.02 inch (about 0.05 cm) to about12 inches (about 30 cm) and, for example, a size in a range from about 2inches to about 6 inches. The pressure, as embodied by the invention, isin a range from greater than about 5 kbar to about 80 kbar, and thetemperature for the gallium nitride crystal growth process is in a rangebetween about 550° C. and about 3000° C. The GaN single crystals thusformed are substantially transparent, with an absorption coefficientbelow 100 cm⁻¹. Furthermore, the substrates of the present inventionhave carrier mobilities above about 100 cm²/V-s and strain, with respectto undoped GaN homoepitaxial layers, below about 0.005%.

After being held at high temperature and high pressure for the desiredperiod, the HPHT system is allowed to cool and the high pressure isrelieved. The gallium nitride crystals are then removed from the HPHTsystem and pressure cell and washed in water and mineral acids. Themineral acids for washing the gallium nitride crystals include, but arenot limited to, hydrochloric acid (HCl) and nitric acid (HNO₃).

The mineralizers, as embodied by the invention, comprise at least one ofalkali, alkaline-earth, and rare earth nitrides such as, but not limitedto: at least one of Li₃N, Mg₃N₂, and Ca₃Na₂; amides, such as LiNH₂,NaNH₂, and KNH₂; urea and related compounds; ammonium salts, such asNH₄F and NH₄Cl; halide, sulfide, and nitrate salts, such as NaCl, CeCl₃,Li₂S, and KNO₃; lithium salts; and combinations thereof. Themineralizers may be provided as solids or as additives dissolved influids, such as solvents. The use of alkaline-earth or rare-earthmineralizers have the additional advantage of acting as a getter foradventitious oxygen in the growth medium, allowing for the growth ofundoped GaN crystals with low n-type carrier density. Alternatively, themineralizer can be formed in situ. At least one of lithium, sodium,potassium, rubidium, cesium, magnesium, calcium, strontium, barium or arare-earth metal may be provided, which then reacts with the ammoniasolvent to form the mineralizer.

The filling and sealing steps will now be described. The capsule isfilled with a nitrogen-containing solvent, such as at least one ofammonia or hydrazine or an organic solvent, including, but not limitedto methylamine, melamine, or ethylenediamine, without admitting air orwater, which are undesirable in the gallium nitride formation process.To fill the capsule without admitting air or water, the capsule isfilled and connected to a negative pressure source, such as a vacuummanifold, and evacuated. The capsule is then chilled to a temperaturebelow room temperature (preferably about −72° C. or below) andvapor-phase solvent can be admitted to the manifold. The vapor-phasesolvent then condenses in the capsule. For example, if thenitrogen-containing solvent comprises ammonia, the condensation can beperformed at dry ice or liquid-nitrogen temperatures.

The capsule can then be isolated so as to seal the capsule by closing avalve to the negative pressure source. The capsule can then be separatedfrom at least one of the manifold or the valve by a pinching-off stepusing a cold welding apparatus, which is well known in the art. Thepinching-off step is particularly effective if the capsule is copper.The integrity of the seal may be enhanced by optional arc welding.

The capsule and pressure cell comprise any appropriate form that permitsthe gallium nitride growth process to withstand the high pressure andhigh temperature as embodied by the invention. The HPHT system thatapplies the high pressures and high temperatures can comprise a pressdevice, which may include at least one of a die and punch. For example,the press device comprises one of: a piston-cylinder press; a beltpress; a tetrahedral-, cubic-, or octahedral-anvil press; arecessed-anvil press; a split-sphere press; and a toriod-type press,each of which are known to those of skill in the art.

The foregoing description of the process for forming the GaN crystalsubstrate is intended to be illustrative only, and should not beconstrued in any limiting sense. Other methods for forming the crystalwill be obvious to those skilled in the art, but are intended to fallwithin the scope of the present disclosure.

The GaN crystal formed is of high quality as determined by a measurementof dislocation density. The dislocation density is determined byperforming transmission electron microscopy (TEM) on a thin section, asis well known in the art. A GaN crystal of the immediate inventioncontains less than about 10⁵ threading dislocations per cm² and,preferably, less than about 10³ dislocations per cm².

After the crystal has been formed, the substrate for the devicefabrication is cut from a boule formed by the method described above.The wafer may either comprise n-type GaN, with an electrical resistivityless than about 1000 Ω-cm, more preferably less than about 100 Ω-cm, oreven more preferably less than about 10 Ω-cm, or insulating GaN, havinga resistivity of at least about 10⁵ Ω-cm. The substrate is then polishedto a mirror finish using mechanical-polishing techniques that are wellknown in the art. Subsurface damage may remain after the polishingprocess. This damage may be removed by several methods that are known inthe art, including chemically assisted ion beam etching orchemo-mechanical polishing. The residual damage may also be removed byheating the wafer to a temperature between about 900 and 1500° C. in anatmosphere containing ammonia at a partial pressure between about 10–8mbar and 20,000 bar. The substrate preferably has a thickness betweenabout 0.01 and 10 mm, most preferably between about 0.05 and 5 mm.

The wafer used in the present invention preferably has a gallium nitridewurtzite-type crystal structure. Moreover, the GaN wafers have a (0001)crystallographic orientation, preferably with a Ga-terminated (0001)face and an N-terminated (000 1) face. It is expected that the (0001) Gaface will be superior for deposition of photodetector device structures.

In the exemplary embodiment shown in FIG. 2, the photodetector 100 has ametal-semiconductor-metal (MSM) structure having at least one activelayer 104. The at least one active layer 104 is an insulating layerdisposed on a surface of substrate 102 and, in one embodiment, comprisesGa_(1-x-y)Al_(x)In_(y)N_(1-z-w)P_(z)As_(w), wherein 0≦x, y, z, w≦1,0≦x+y≦1, and 0≦z+w≦1. In another embodiment, the at least one activelayer 104 comprises Ga_(1-x)Al_(x)N, wherein 0≦x≦1. The insulating layer104 can be doped or undoped, and typically has a thickness in the rangebetween about 1 nm to about 10 microns. Additionally, insulating layer104 typically has a carrier concentration of up to about 10¹⁸ cm⁻³.Substrate 102 comprises either n-doped or insulating gallium nitride.Conductive contact structure 106, comprising a plurality of Schottkycontacts 108, is disposed on a surface 110 of insulating layer 104. Asshown in FIG. 2, Schottky contacts 108 are interdigitated with respectto each other. Typically, Schottky contacts 108 are made of nickel andgold. A portion of a respective Schottky contact 108 that contactsinsulating layer 104 is preferably a contact layer (not shown)comprising at least one of nickel and a nickel-rich nickel-goldcomposition. Typically, the contact layer is contacted with at least oneof gold and a gold-rich nickel-gold composition.

Metallic contacts are good electrical conductors, but have thedisadvantage of having poor optical transparency, which decreases thelight-collecting efficiency of the photodetector. This can be overcomeby using conductive metal oxides such as, but not limited to, tin oxideand indium oxide instead of, or in combination with, the correspondingmetal. Among the materials that may be used as Schottky or ohmiccontacts are palladium, platinum, gold, aluminum, tin, indium, chromium,nickel, titanium, and oxides thereof. Additional materials that may beused as ohmic contacts include, but are not limited to, scandium,zirconium, tantalum, tungsten, copper, silver, hafnium, and rare earthmetals.

In the embodiment shown in FIG. 2, Schottky contacts 108 may besputtered onto surface 110 of insulating layer 104. Alternatively,Schottky contacts 108 may be deposited onto surface 110 of insulatinglayer 104 by electron beam evaporation. While sputtering and electronbeam evaporation are discussed here, these processes are not to beconsidered limitations of the instant invention. In fact, any equivalentprocess can be used to deposit Schottky contacts 108 onto surface 110.In another embodiment, an n-doped layer 112 is disposed betweensubstrate 102 and insulating layer 104.

In the exemplary embodiment shown in FIG. 3, the photodetector 200 has aP-i-N structure which includes a n-doped substrate 202, an insulatinglayer 204 disposed on a surface 206 of n-doped substrate 202 and a firstp-doped layer 208 disposed on a surface 209 of insulating layer 204opposite n-doped substrate 202, as shown in FIG. 3. Insulating layer 204and first p-doped layer 208 each have a nominal thickness in the rangebetween about 1 nm to about 10 microns. Insulating layer 204 has anominal carrier concentration of up to about 10¹⁸ cm^(−3.)

A conductive contact structure 210 typically comprises a first ohmiccontact 212, typically made of nickel and gold. The first ohmic contact212 is affixed to said the first p-doped layer 208 and a second ohmiccontact 214, typically made of titanium and aluminum, is affixed to then-type substrate 202.

A portion of the first ohmic contact 212 that contacts first p-dopedlayer 208 is a contact layer 216 made of at least one of nickel and anickel-rich nickel-gold composition. Typically, the contact layer 216 iscontacted with an overlayer 217 comprising at least one of gold and agold-rich nickel-gold composition. Among the materials that may be usedas the first ohmic contact 212 to the first p-doped layer 208 arepalladium, platinum, gold, aluminum, tin, indium, chromium, nickel,titanium, and oxides thereof. It is understood that these materials maybe used to form an ohmic contact with any of the p-doped layersdescribed herein.

In the embodiment shown in FIG. 3, the first ohmic contact 212 maysputtered onto a surface 211 of the first p-doped layer 208.Alternatively, the first ohmic contact 212 may be deposited onto surface211 of the first p-doped layer 208 by electron beam evaporation. Whilesputtering and electron beam evaporation are discussed here, theseprocesses are not to be considered limitations of the instant invention.In fact, any equivalent process can be used to deposit first ohmiccontact 212 onto surface 211.

In the embodiment shown in FIG. 3, a portion of the second ohmic contact214 that contacts n-type substrate 202 is preferably a contact layer 218typically comprising a titanium-rich titanium-aluminum composition.Typically, the contact layer 218 is contacted with an overlayer 219having an aluminum-rich titanium-aluminum composition. Materials thatmay be used as the second ohmic contact 214 that contacts n-typesubstrate 202 include, but are not limited to, aluminum, scandium,titanium, zirconium, tantalum, tungsten, nickel, copper, silver, gold,hafnium, and rare earth metals. It is understood that these materialsmay be used to form an ohmic contact with any of the n-doped layersdescribed herein.

In the embodiment shown in FIG. 3, the second ohmic contact 214 may besputtered onto n-type substrate 202. Alternatively, second ohmic contact214 may be deposited onto the n-type substrate 202 by electron beamevaporation. While sputtering and electron beam evaporation arediscussed here, these processes are not to be considered limitations ofthe instant invention. In fact, any equivalent process can be used todeposit second ohmic contact 214 onto n-type substrate 202.

In the embodiment shown in FIG. 3, photodetector 200 may furthercomprise a second p-doped layer 220 comprising, for example, p-dopedaluminum gallium nitride, disposed on a surface 211 of the first p-dopedlayer 208 opposite insulating layer 204. The photodetector may furthercomprise an n-doped layer 224, comprising, for example, n-doped galliumnitride, disposed between n-doped substrate 202 and insulating layer204. In one embodiment of the invention shown in FIG. 3, insulatinglayer 204, first p-doped layer 208, second p-doped layer 220, andn-doped layer 224 each comprise Ga_(1-x)Al_(x)N, wherein 0≦x≦1. Inanother embodiment, insulating layer 204, first p-doped layer 208,second p-doped layer 220, and n-doped layer 224 each compriseGa_(1-x-y)Al_(x)In_(y)N_(1-z-w)P_(z)As_(w), wherein 0≦x, y, z, w≦1,0≦x+y≦1, and 0≦z+w≦1.

In the exemplary embodiment shown in FIG. 4, the photodetector 300 is aSchottky barrier device in which at least one active layer 302 comprisesan insulating layer disposed on a surface 304 of a substrate 306, whichis typically either an n-doped or insulating GaN substrate, and aconductive contact structure 308 comprising at least one Schottkycontact 310, typically made of nickel and gold, affixed to insulatinglayer 302 and at least one ohmic contact 312, typically made of titaniumand aluminum, is affixed to substrate 306. The insulating layer 302 hasa nominal carrier concentration of up to about 10 ¹⁸ cm³.

In the embodiment shown in FIG. 4, photodetector 300 may furthercomprise a first n-doped layer 314 disposed between substrate 306 andinsulating layer 302. The first n-doped layer 314 has a nominalthickness in the range between about 1 nm to about 10 microns. Inanother embodiment, photodetector 300 may further comprise a secondn-doped layer 316, typically comprising n-doped gallium nitride, that isdisposed between substrate 306 and first n-doped layer 314. In thisembodiment, the substrate 306 is typically an insulating GaN substrate.The second n-doped layer 316 contacts at least one ohmic contact 312.Second n-doped layer 316 has a nominal thickness in the range betweenabout 1 nm to about 10 microns. In one embodiment of the invention shownin FIG. 4, active (which, in this example, is insulating) layer 302,first n-doped layer 314, and second n-doped layer 316 each compriseGa_(1-x)Al_(x)N, wherein 0≦x≦1. In another embodiment, active (which, inthis example, is insulating) layer 302, first n-doped layer 314, andsecond n-doped layer 316 each compriseGa_(1-x-y)Al_(x)In_(y)N_(1-z-w)P_(z)As_(w), wherein 0≦x, y, z, w≦1,0≦x+y≦1, and 0≦z+w≦1.

A portion of at least one Schottky contact 310 that contacts insulatinglayer 302 is preferably a contact layer 318 that comprises at least oneof nickel and a nickel-rich nickel-gold composition. Typically, contactlayer 318 is contacted with an overlayer 319 comprising at least one ofgold and a gold-rich nickel-gold composition. At least one Schottkycontact 310 has a nominal thickness in the range between about 0.001microns to about 10 microns.

In the embodiment shown in FIG. 4, the at least one Schottky contact 310may be sputtered onto insulating layer 302. Alternatively, Schottkycontact 310 may be deposited onto insulating layer 302 by electron beamevaporation. While sputtering and electron beam evaporation arediscussed here, these processes are not to be considered limitations ofthe instant invention. In fact, any equivalent process can be used todeposit Schottky contact 310 onto insulating layer 302.

A portion of at least one ohmic contact 312 that contacts substrate 306is a contact layer 320, preferably made of a titanium-richtitanium-aluminum composition. Typically, contact layer 320 is contactedwith an overlayer 321 having an aluminum-rich titanium-aluminumcomposition.

In one embodiment, at least one ohmic contact 312 is sputtered ontosubstrate 306. Alternatively, ohmic contacts 312 are deposited ontosubstrate 306 by electron beam evaporation. While sputtering andelectron beam evaporation are discussed here, these processes are not tobe considered limitations of the instant invention. In fact, anyequivalent process can be used to deposit ohmic contacts 312 ontosubstrate 306.

In one embodiment, at least one of substrate 102, 202, 306 and at leastone of active layer 104, 204, 302 further comprise at least onen-dopant, typically selected from the group consisting of silicon,germanium, and oxygen. The n-dopant is typically epitaxially depositedin at least one of substrate 102, 202, 306 and at least one active layer104, 204, 302. Alternatively, the n-dopant is implanted in at least oneof substrate 102, 202, 306 and at least one active layer 104, 204, 302.

In another embodiment of the present invention, at least one ofsubstrate 102, 202, 306 and at least one of active layer 104, 204, 302further comprise at least one p-dopant, typically selected from thegroup consisting of magnesium, calcium, and beryllium. The p-dopant istypically epitaxially deposited in at least one of substrate 102, 202,306 and at least one active layer 104, 204, 302. Alternatively, p-dopantis implanted in at least one of substrate 102, 202, 306 and at least oneactive layer 104, 204, 302.

In one embodiment, photodetector 100, 200, 300 is used in a flamedetector adapted to detect a flame in a combustion chamber (not shown).The stoichiometry of each of the active layers 104, 204, 302 determinesthe sensitivity of respective photodetector 100, 200, 300 to particularwavelengths of electromagnetic radiation. More specifically, therelative amounts of the different metals in the active layer—forexample, the relative amounts of aluminum and gallium in Ga_(1-x)Al_(x)N—determine the wavelength range to which the photodetector 100, 200, 300will respond. Photodetector 100, 200, 300 can thus be tuned to detectspecific wavelengths of radiation by depositing an active layer 104,204, 302 having the appropriate composition. A combination of at leasttwo flame detectors may be used to monitor two different emission rangesfor flame temperature determination. In another embodiment, substrate102, 202, 306 is a gallium nitride substrate comprising a single crystalgallium nitride wafer and having a nominal dislocation density of lessthan about 10⁵ cm⁻². Gallium nitride substrate 102, 202, 306 has anominal resistivity of at least about 10⁵ Ω-cm. Alternatively, thegallium nitride substrate 102, 202, 306 has a resistivity of less thanabout 10 Ω-cm. Gallium nitride substrate 102, 202, 306 preferably has anominal dislocation density of less than about 10³ cm^(−2.)

The gallium nitride wafer has a nominal diameter in the range betweenabout 3 mm to about 150 mm. Preferably, the gallium nitride wafer has adiameter in the range between about 12 mm and about 150 mm. Mostpreferably, the gallium nitride wafer has a diameter in the rangebetween about 20 mm to about 150 mm. The gallium nitride wafer typicallyhas a (0001) crystallographic orientation.

A method 400 of making a photodetector 100, 200, 300, wherein thephotodetector 100, 200, 300 comprises a gallium nitride substrate 102,202, 306, at least one active layer 104, 204, 302 disposed on galliumnitride substrate 102, 202, 306 and at least one conductive contactstructure 106, 210, 308 affixed to at least one of gallium nitridesubstrate 102, 202, 306 and active layer 104, 204, 302, is shown in theflow chart of FIG. 5.

Method 400 comprises the steps of: 402 providing a gallium nitridesubstrate (102, 202, 306); 404 depositing at least one active layer(104, 204, 302) on the gallium nitride substrate (102, 202, 306); and406 affixing a conductive connecting structure (106, 210, 308) to atleast one of the at least one active layer (104, 204, 302) and thegallium nitride substrate (102, 202, 306).

Step 404, which comprises depositing at least one active layer (104,204, 302) on the gallium nitride substrate (102, 202, 306), typicallycomprises depositing at least one active layer (104, 204, 302) by metalorganic vapor phase epitaxy or, alternatively, by molecular beamepitaxy.

Step 406, which comprises affixing a conductive connecting structure106, 210, 308 to at least one of the active layer (104, 204, 302) andgallium nitride substrate (102, 202, 306) may include eithersputter-depositing a metallic layer on at least one of the at least oneactive layer (104, 204, 302) and the gallium nitride substrate (102,202, 306) or, alternatively, electron beam evaporating a metallic layeron at least one of the at least one active layer (104, 204, 302) and thegallium nitride substrate (102, 202, 306).

In one embodiment, method 400 further includes the step 408 ofincorporating at least one dopant into the gallium nitride substrate(102, 202, 306).

Step 408, which comprises incorporating at least one dopant into thegallium nitride substrate (102, 202, 306), may comprise epitaxiallydepositing a doped layer on the gallium nitride substrate (102, 202,306), preferably by metal organic vapor phase epitaxy. Alternatively,the dopant may be incorporated into the gallium nitride substrate (102,202, 306) by implanting the dopant in the gallium nitride substrate(102, 202, 306).

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A gallium nitride substrate for a photodectector, said galliumnitride substrate comprising a homoepitaxially grown single crystalgallium nitride wafer and having a dislocation density of less thanabout 10⁵ cm², wherein said gallium nitride substrate has a dislocationdensity of less than about 10³ cm⁻².
 2. The gallium nitride substrate ofclaim 1, wherein said gallium nitride substrate has a resistivity of atleast about 10⁵ Ω-cm.
 3. The gallium nitride substrate of claim 1,wherein said gallium nitride substrate has a resistivity of less thanabout 10 Ω-cm.
 4. The gallium nitride substrate of claim 1, wherein saidgallium nitride wafer has a diameter of between about 3 mm and about 150mm.
 5. The gallium nitride substrate of claim 4, wherein said galliumnitride wafer has a diameter of between about 12 mm and about 150 mm. 6.The gallium nitride substrate of claim 5, wherein said gallium nitridewafer has a diameter of between about 20 mm and about 150 mm.
 7. Thegallium nitride substrate of claim 1, wherein said gallium nitride waferhas a (0001) crystallographic orientation.
 8. A gallium nitridesubstrate for a photodetector, said gallium nitride substrate comprisinga homoepitaxially grown single crystal gallium nitride water and havinga dislocation density of less than about 10⁵ cm², wherein said galliumnitride wafer is a gallium nitride wafer cut from a boule that was grownusing a supercritical solvent at a temperature greater than about 550°C. and a pressure greater than about 5 kbar.
 9. A gallium nitridesubstrate for a photodetector, said gallium nitride substrate comprisinga homoepitaxially grown single crystal gallium nitride wafer having adislocation density of less than about 10³ cm⁻² cut from a portion of aboule grown by precipitating gallium nitride onto at least one of agallium nitride crystal, a gallium nitride boule, and a gallium nitridecrystal seed using a supercritical solvent at a temperature greater thanabout 550° C. and a pressure greater than about 5 kbar.
 10. A galliumnitride substrate for a photodetector, said gallium nitride substratecomprising a homoepitaxially grown single crystal gallium nitride waferhaving a dislocation density of less than about 10⁵ cm⁻² cut from aportion of a boule grown by precipitating gallium nitride onto at leastone of a gallium nitride crystal, a gallium nitride boule, and a galliumnitride crystal seed using a supercritical solvent at a temperaturegreater than about 550° C. and a pressure greater than about 5 kbar. 11.A photodetector, comprising: the gallium nitride substrate as defined inclaim 2, wherein at least one active layer is disposed on the galliumnitride substrate, and the at least one active layer comprisingGa_(1-x-y)Al_(x)In_(y)N_(1-z-w)P₂As_(w); wherein 0 less than or equal tox, y, z, w less than or equal to 1, 0 less than or equal to x+y lessthan or equal to 1; and 0 less than or equal to z+w less than or equalto 1; and at least one conductive contact structure is secured to thegallium nitride substrate and to the at least one active layer.
 12. Thephotodetector as defined in claim 11, wherein the gallium nitridesubstrate Is a wafer and has a (0001) crystallographic orientation. 13.The photodetector as defined in claim 11, wherein the gallium nitridesubstrate is a wafer and has a diameter in a range of from about 3 mm toabout 150 mm.
 14. The photodetector as defined in claim 11, wherein thephotodetector defines a P-I-N structure.
 15. The photodetector asdefined in claim 14, wherein the photodetector comprises an n-dopedsubstrate, and further comprising an insulating layer disposed on asurface of the n-doped substrate.
 16. The photodetector as defined inclaim 15, further comprising a p-doped substrate disposed on a surfaceof the insulating layer opposite the n-doped layer.
 17. Thephotodetector as defined in claim 16, wherein the insulating layer orthe p-doped layer each have a nominal thickness in a range of from about1 nanometer to about 10 micrometers.